Safety Not Guaranteed sounds incredibly gimmicky at first — a whole movie based on an internet meme. Someone placed an ad (in real life) in a magazine in 1997, saying "Wanted: Someone to Go Back in Time With Me." And now, it's a movie. You could easily imagine this turning into something annoyingly quirky and self-satisfied.

So it's an amazing surprise that Safety Not Guaranteed turns out to be a powerful emotional journey. Even with some flaws here and there, this is one of the most effective movies we've seen lately.

If the measure of a great movie is how much it keeps coming back into your mind in the days after you've seen it, then Safety Not Guaranteed is pretty great. There's a lot to debate here about what some of the film's clever twists and revelations, and a few things are kind of left up for interpretation. But also, the characters in this film have a way of popping back into your head. There are a few really rich, layered characters in this movie, and it's a film with something to say about our relationship with the past.

We've seen a lot of fusions of the Sundance aesthetic with science fictional ideas in recent years, most notably the twofilms starring and co-written by Brit Marling. The ultra-personal, indie-rock sensibility of Sundance obviously blends really well with an off-kilter look at a speculative conceit, and we've liked a lot of these films. But few of them have seemed like they knew what they wanted to say about their science fictional idea as much as Safety Not Guaranteed.

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Fundamentally, this is a movie about time travel, which uses the notion of a time machine both as a metaphor and as a potential plot device. It's a film about regret and nostalgia, and the fact that everybody has stuff in their past that they'd like to "fix" or return to. This is a great universal theme, and the film manages to get some new stuff out of it, without ever leaning on it too hard or patting itself on the back.

In the Q&A after the film screening, director Collin Trevorrow compared his approach to time travel to Back to the Future or Eternal Sunshine of the Spotless Mind — both of those movies are about engaging with the past in a very personal, intimate way. (We'll have our own exclusive interview with Trevorrow next week.)

Oh, and this film is massively funny in parts — but it's not a zany comedy about lovable misfits. It's actually quite dark in parts, and surprisingly moving.

Spoilers ahead...

In Safety Not Guaranteed, Aubrey Plaza (Parks & Rec) plays Darius, a young intern at Seattle Weekly magazine who hates her meaningless life. She's assigned to go to the middle of nowhere to help with an article about the mysterious magazine ad from a time-traveler seeking a companion. Darius is accompanying Jeff (New Girl's Jake M. Johnson) and a fellow intern Arnau (Karan Soni) to go find out the truth about this so-called time traveler. But when she meets the guy who placed the ad, Kenneth (Mark DuPlass), she finds herself bonding with him over their shared loneliness and regrets.

One cool thing about Safety Not Guaranteed is that it does manage to keep the question of "Is Kenneth a time traveler or just crazy" alive for the whole 90 minutes, without ever resorting to the kind of gimmicky fake ambiguity that a lot of these sorts of movies rely on. (And yes, there is a definitive answer. The film does not try to have it both ways in the end.)

Another cool thing? The film has a really neat structure, which amplifies the theme of dealing with the past and confronting your regrets. Jeff, Darius' boss, has an old high-school flame who lives in the same small town as Kenneth the supposed time traveler. Jeff seeks out Liz, his old sweetheart, only to find out that she's gained weight and looks her age (the same age he is, in other words.) At first, Jeff is repulsed — but then he gives Liz another chance, and finds himself falling in love with her all over again, replacing the image he's kept in his head for the past 18 years with the living, breathing woman.

Jeff's uncertain quest to reconnect with a woman from his past is juxtaposed with Kenneth's quest to go back in time and change a tragedy that happened to someone he loved. And as the movie goes on, we learn that neither of these things is as straightforward as they first appear — because people are complicated and messed up. That's where this film really starts to take a hold of your brain, especially as you realize that Jeff and Kenneth are both even more broken than they first appear.

And yes, this is a very, very indie rock movie. So much so, that Kenneth the time-travel guy even writes an indie rock song that he wants to go back in time and perform for his lost love. You can hear an acoustic performance of it by the film's scorer, at left.

Safety Not Guaranteed is definitely not a perfect film — there's a love story that seems to come very much out of left field, the character of Arnau the intern is a bit of a one-dimensional stereotype, a few things that happen seem a bit random — but it's a really fun, super-engaging film. We all clapped and cheered our heads off at the ending, at the screening I was at.

And all in all, this is one of those rare science fiction films that takes both its characters and its science fiction seriously — and it tells a good freaking story along the way. You owe it to yourself to check out Safety Not Guaranteed.

Safety Not Guaranteed is out this weekend in New York and Los Angeles, and more cities next weekend.